[OH:1][C:2]1[CH:7]=[CH:6][C:5]([C:8]2[O:17][C:12]3=[N:13][CH:14]=[CH:15][CH:16]=[C:11]3[C:10](=[O:18])[CH:9]=2)=[CH:4][CH:3]=1.[CH3:19][C:20](OC(C)=O)=[O:21].O>CN(C1C=CN=CC=1)C.N1C=CC=CC=1>[C:20]([O:1][C:2]1[CH:3]=[CH:4][C:5]([C:8]2[O:17][C:12]3=[N:13][CH:14]=[CH:15][CH:16]=[C:11]3[C:10](=[O:18])[CH:9]=2)=[CH:6][CH:7]=1)(=[O:21])[CH3:19]In a 50 mL round-bottomed flask fitted with condenser and magnetic stirrer were placed 2-(4-hydroxy-phenyl)-pyrano[2,3-b]pyridin-4-one (1.5 g, 6.27 mmol), Ac2O (700 mg, 6.89 mmol) and a catalytic amount of DMAP (50 mg) in pyridine (10 mL) and the reaction mixture was stirred for 16 h at rt. The reaction mixture was poured into water and extracted with ethyl acetate. The organic layer washed with water, dried and concentrated to give a 2-(4-acetoxyphenyl)-pyrano[2,3-b]pyridin-4-one (930 mg, 53%). To a stirred solution of the 4-acetoxy compound (930 mg, 3.30 mmol) in CH2Cl2 (50 mL), mCPBA(1.9 g, 11.3 mmol) was added and the reaction mixture was stirred at rt for 5 days. The reaction mixture was diluted with CH2Cl2 and washed with NaHCO3 solution. The organic layer was separated, washed with water, brine, dried and concentrated to give crude product, which was purified by column chromatography, using 5% methanol in CH2Cl2 to give 168 mg of N-oxide product in 17% yield. To a solution of the acetoxy N-oxide (168 mg, 0.565 mmol) in methanol: THF (10 mL: 10 mL), K2CO3 (156 mg, 1.13 mmol) was added and the mixture was stirred for 2 h at rt. The reaction mixture was neutralized by dilute HCl. The solid was isolated by filtration, washed with water and dried to give 2-(4-hydroxyphenyl)-4-oxo-4H-pyrano[2,3-b]pyridine 8-oxide (150 mg, 100%). MS (ES) m/z: 255.90 (M); Mp. 314-315° C.
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